Flowmeters which are operated in a magnetically inductive manner are known. In this case, a magnetic field is induced via a defined measuring tube, and a signal generated by the magnetic field is measured using at least one pair of electrodes having a junction with the fluid.
Methods and devices of this type have recently been used with diagnostic means for detecting the state of the device, on the one hand, and for detecting disruptions in the flow of the measuring medium, on the other hand. To this end, the actual measurement signal which determines the flow rate is used only secondarily. It is primarily the arrangement of electrodes, which are in contact with the measuring medium, that is used to electrically obtain significant measured values. When signals are fed in via the electrodes, limit values which reliably prevent electrolysis processes from being initiated at the electrodes in the measuring medium are technically set with respect to voltage and current.
However, in addition to the functional disruptions of the device itself, otherwise uniform flows of the measuring media may also enter disrupted states, for example on account of cavitation in which turbulent flows past tube contours produce such negative pressures that gas bubbles, which then also have to pass through the flowmeter device, are suddenly nevertheless produced in a fluid medium which is otherwise free of gas bubbles. Furthermore, other materials such as solids, contaminants or the like in the measuring medium may cause disruptions such that, in a manner similar to gas bubbles, although a sum flow rate is measured, it does not exclusively correspond to the flow rate of the actually desired measuring medium. It proves to be particularly difficult to detect this state, in particular.
In addition, flowmeters are subject to ageing processes which have a drifting influence on the flow rates determined. In other words, the susceptibility to faults increases.
In this case, it is now particularly important to be able to preclude sources which give rise to conventional faults. These are the gas bubbles and particles in the measuring medium which have already been described above.
Therefore, in order to detect aging effects, effects produced by gas bubbles and particles must be clearly identified as such first of all.
DE 102 43 748 and DE 101 18 002 disclose electromagnetic flowmeters in which impedances between one or more electrodes and an earthing point are measured. However, gas bubbles in the flow, for example, are not detected in a significant manner in this case.